"I am on a healthy diet."
Translation:Jestem na zdrowej diecie.
Being on a diet seems like a rather idiomatic thing in English, I'm surprised it works just the same in Polish.
Would 'dieta' be accusative in a sentence like "I'm going on a diet": "...na dietę"?
so in English "diet" means both the stuff you eat and also losing weight, is this the same in Polish too?
In Polish - dieta technically means what you eat. It can also mean a specific way of eating that is not common - diet in a specific condition, for a specific purpose -one of those purposes is a weight loss diet (dieta odchudzająca). Weight loss diet is so "common" that in many circumstances "dieta" without any adjective is assumed to be "odchudzająca".
but losing weight= odchudzanie (noun), odchudzać się (verb).
Also dieta is a name of payment for some jobs (like in parliament, or city council), and compensation for some expenses while on a work-related travel.